1. Field of the Invention
The present invention relates to nano-diamonds, and in particular relates to surface modification of the nano-diamonds.
2. Description of the Related Art
There are three commercial methods to produce nano-diamonds: Chemical vapor deposition, high pressure catalysis, and detonation. Different manufacturing methods result in different forms of the nano-diamonds, wherein the forms of the nano-diamonds can be commonly divided into ultradisperse nano-diamonds (UDD), monocrystalline nano-diamonds, and polycrystalline nano-diamonds. The ultradisperse nano-diamonds have particle sizes between 4 to 6 nm, and surfaces of the ultra disperse nano-diamonds are covered by a fullerene-like carbon, which aggregates into particles of hundreds of nanometers in diameter. Both the monocrystalline nano-diamonds and the polycrystalline nano-diamonds are single particle nano-diamonds with no graphite structures on surfaces thereof. The nano-diamonds are not only hard, they also have extremely high thermal conductivity, high wear-resistance, and good chemical stability, but they also have large surface areas and high surface activities. Nano-diamonds have been proposed to be used as a lubricating material (Chepovetskii, I. Kh.; et al. Mater. 1993, 3, 48-50), as reinforce additive of high strength resins or rubbers (Chkhalo, N. I.; et al. NIST Spec. Publ., 1995, 88, 27-30), for high precision polishing (Dolmatov, V. Y. Russ. Chem. Rev., 2001, 70, 607.), as a wear resistant composite film (Chang, et. al., China Patent Application No 200710074533.4), and so forth. It is often desirable to improve the dispersion of the nano-diamonds in solvents in order to increase their applicability. However, nano-diamonds easily aggregate to micro size, and lose their unique features as nano-particles due to the high specific surface energy. Therefore, nano-diamonds have not been widely used in these fields. Currently, another reason why nano-diamonds have not been widely used is that it is difficult to improve the dispersion of the nano-diamonds in media, while at the same time increase the stability of the dispersion; especially for the monocrystalline nano-diamonds and the polycrystalline nano-diamonds, wherein surface modifications of the trace graphite structure on the surfaces thereof are not easy. Note that surface modification or dispersion techniques related to nano-diamonds have been disclosed, such as US 2008/0249229A1, which utilizes atom-transfer-radical polymerization (ATRP) to graft polymers onto the surface of nano-diamonds. The technique requires using nitric acid to oxidize the graphite structure on the surface of the nano-diamonds into carbonate. Then, a series of reactions proceed to graft polymers onto the nano-diamonds, and polymer-grafted nano-diamonds are then dispersed. Although the method can be used on ultra disperse nano-diamonds successfully, it is difficult to apply the method to monocrystalline nano-diamonds or polycrystalline nano-diamonds, which have larger sizes (>50 nm) and no surface graphite structures. Since the method requires oxidizing the graphite structure to facilitate polymer grafting, nano-diamonds without the surface graphite structure can not be grafted with polymers to stabilize the dispersion. Also, because the method requires multiple steps for chemical reactions and purifications, it is unfavorable for industrial applications. In China Patent Application No. 02115230.6, nano-diamonds were modified by the specific silane reagent. Although the method improved the stability of the nano-diamonds in medium, the cost of the silane reagent is high, and the reaction time is long, thus, limiting industrial applications. In another example such as China Patent Application No. 02139764.3, surfactant was added into nano-diamonds by gas flow pulverization, high pressure liquid flow pulverization, or bead milling. By physical pulverization or mechanical milling, the nano-diamonds were equally dispersed into solution. However, because the surfactant is absorbed on the surface of the nano-diamonds, the nano-diamonds can only be dispersed into some specific solvent, and therefore, the applications thereof are limited. Currently, all surface modification techniques of nano-diamonds focus on ultra disperse nano-diamonds, and there is no effective dispersion technique for monocrystalline or polycrystalline nano-diamonds, which have no graphite structure thereon. It is highly desirable to develop an effective and rapid technique to functionalize a surface of the nano-diamonds and to stably disperse the monocrystalline or the polycrystalline nano-diamonds into solvents. Thus, the invention provides a rapid and economically effective method to homogenously graft a lot amount of polymers onto surfaces of nano-diamonds. Moreover, all kinds of nano-diamonds can be modified, and the nano-diamonds can be uniformly dispersed in different solvents in high concentration.